Many computing systems include a power-fail system for detecting a power failure and transitioning the computing system from system power to battery power. Typically, the power-fail system places the memory system in an auto refresh mode to retain stored data.
Two factors that effect effective data retention is the speed at which the system enters the retention state and the ability to remain in that state. Most events that place the memory into the self-refresh state are asynchronous and unpredictable. Examples of these events include depressing the reset button the failure of a power supply. Neither of these situations has a predictable duration, and therefore the memory sub-system must be placed in the self-refresh state.